Recently, as a method for producing an SOI wafer, the method comprising bonding a wafer implanted with hydrogen ions or the like and then delaminating the wafer to produce an SOI wafer (a technique called ion implantation delamination method: Smart Cut Method (registered trademark)) is newly coming to attract much attention. This method is a technique for producing an SOI wafer, wherein an oxide film is formed on at least one of two silicon wafers, gaseous ions such as hydrogen ions or rare gas ions are implanted into one wafer (bond wafer) from its top surface to form a micro bubble layer (enclosed layer) in this silicon wafer, then the ion-implanted surface of the wafer is bonded to the other silicon wafer (base wafer) via the oxide film, thereafter the wafers were subjected to a heat treatment (delamination heat treatment) to delaminate one of the wafers (bond wafer) as a thin film at the micro bubble layer as a cleavage plane, and the bonded wafer is further subjected to a heat treatment (bonding heat treatment) for firm bonding to obtain an SOI wafer (refer to Japanese Patent Laid-open (Kokai) Publication No. 5-211128). In this method, the cleavage plane (delaminated plane) is obtained as a good mirror surface, and an SOI wafer also having high uniformity of the thickness of the SOI layer is comparatively easily obtained.
However, when an SOI wafer is produced by the ion implantation delamination method, a damaged layer remains on the surface of SOI wafer after delamination due to the ion implantation, and the surface has higher surface roughness compared with mirror surfaces of silicon wafers of usual product level. Therefore, it becomes necessary to remove such a damaged layer and roughening of the surface in the ion implantation delamination method. Conventionally, in order to remove the damaged layer and so forth, mirror polishing using a small amount of stock removal for polishing (stock removal: about 100 nm), which is called touch polish, has been performed in a final step after the bonding heat treatment.
However, if the SOI layer is subjected to polishing involving a mechanical processing factor, there is caused a problem that uniformity of the SOI layer thickness attained by implantation of hydrogen ions or the like and the delamination is degraded, because the stock removal for polishing is not uniform.
Therefore, Japanese Patent Laid-open (Kokai) Publication No. 10-242154 proposed a method for improving the surface roughness by subjecting an SOI wafer obtained by the ion implantation delamination method to a heat treatment in an active atmosphere (hydrogen atmosphere) without polishing the surface of the SOI wafer. It is described that, according to this method, the surface roughness of the SOI layer surface can be improved while maintaining the uniformity of film thickness of the SOI layer.
However, damages resulting from the ion implantation exist in an SOI wafer obtained by the ion implantation delamination method, and the damages in the SOI layer are large at the surface side and become smaller at a deeper position in the layer. Therefore, if the wafer is subjected to such a heat treatment in an active atmosphere as described above, recovery of the damages advances from interior of the SOI layer to the surface side. However, when the damages at the surface side are large, a heat treatment at a high temperature for a long period of time is required, and in addition, complete recovery may not be obtained even if a heat treatment is performed at a high temperature for a long period of time, as the case may be.
Since the strength and depth of the damages are influenced by the amount of implantation energy and dose of gaseous ions such as hydrogen ions, for example, when the implantation energy needs to be increased as in the case of producing an SOI wafer having a thick SOI layer or a thick buried oxide layer, or when the dose needs to be increased for the purpose of performing the delamination heat treatment at a low temperature, the aforementioned problem becomes remarkable.
Furthermore, if the wafer is subjected to a heat treatment at a high temperature for a long period of time under a reducing atmosphere comprising hydrogen gas, silicon at the SOI layer surface may be etched, thus thickness uniformity may be degraded, and etch pits may be formed in a buried oxide layer. This phenomenon is caused by the reason described below. That is, defects such as COPs (Crystal Originated Particles) exist in the SOI layer, and if they are connected to the oxide film as the under layer, COPs do not disappear and remain as they are, or they are even enlarged. Therefore, hydrogen or the like that penetrates through the defects also etches the buried oxide layer, and thus pits are formed there to cause the aforementioned phenomenon. These etch pits are problematic, since they influence also on the SOI layer neighboring the pits.
As described above, although various methods have been proposed in order to remove the damaged layer and roughening of the surface of SOI wafer obtained by the ion implantation delamination method while maintaining the thickness uniformity of SOI layer, there are no satisfactory method so far, and a suitable solution method has been desired.
Therefore, as described in Japanese Patent Laid-open (Kokai) Publication No. 2000-124092, the applicants of the present invention proposed, as a method for producing an SOI wafer of high quality by removing the damaged layer and the roughening of the surface remaining on a surface of SOI layer after the delamination in the ion implantation delamination method while maintaining the thickness uniformity of the SOI layer, a method of forming an oxide film by a heat treatment under an oxidizing atmosphere on the SOI surface after delamination, then removing the oxide film, and subsequently subjecting the wafer to a heat treatment under a reducing atmosphere.
If the so-called sacrificial oxidation is performed, in which an oxide film is formed on the SOI layer by a heat treatment under an oxidizing atmosphere, and then the oxide film is removed, a part of or whole damaged layer on the SOI layer surface can be incorporated into the oxide film, and therefore the damaged layer can efficiently be removed by removing the oxide film. Further, a heat treatment can be subsequently performed under a reducing atmosphere to recover the damaged layer remaining in the SOI layer and improve the surface roughness, and the heat treatment time of the heat treatment under a reducing atmosphere can also be shortened, since a part of or whole damaged layer on the SOI layer surface has been removed by the sacrificial oxidation. Furthermore, this method does not require polishing or the like involving a mechanical processing factor, uniformity of the film thickness of the SOI layer is not degraded, and thus it is considered that an SOI wafer of extremely high quality can be produced by the ion implantation delamination method with higher productivity.
As described above, the technique described in Japanese Patent Publication (Kokai) Laid-open No. 2000-124092 has the advantage that the damaged layer and the roughening of the surface remaining on the SOI layer surface after the delamination can be removed in the ion implantation delamination method while maintaining the thickness uniformity of the SOI layer. However, when the inventors of the present invention performed additional experiments for this technique, it was found that the technique had the following drawbacks and thus the technique was insufficient as it was as a method for production in a large scale.    1) Since the sacrificial oxidation in the aforementioned technique directly oxidizes the delaminated plane obtained by the ion implantation, oxidation induced stacking faults (OSFs) may be generated by the oxidation, and these OSFs may not be completely removed only by the subsequent heat treatment under a reducing atmosphere.    2) Surface roughness sufficiently removed by the heat treatment under a reducing atmosphere mainly consists only of short period components (e.g., period of 1 μm or less), and removal of long period components (e.g., period of about 1 to 10 μm) of the surface roughness may become insufficient.    3) If the high temperature heat treatment is performed under an atmosphere containing a large amount of hydrogen gas as the reducing atmosphere, the hydrogen gas acts on the bonding interface, and thus corrosion of the interface may become significant and thus it may cause particle generation in the device production process.